Process of electro-depositing chromium



Patented July 4, 1933 UNITED STATES PATENT OFFICE WILLIAM S. EATON, OF SAG HARBOR, NEW YORK, ASSIGNOR OF ONE-HALF TO WALTER W. BURNS, OF WASHINGTON, DISTRICT OF COLUMBIA PROCESS OF ELEGTRO-DEPOSITI'NG CHROMI'UM No Drawing.

My invention relates to a process of electrodepositing chromium, and more particularly to a process involving the use of a chromic acid solution.

In the electrolytic depositing of chromium, particularly in the plating of various articles, great difficulties have been experienced, due in part to the varying resistance conditions in the bath during the plating operation, and in part to difficulties in securing a continuous, uninterrupted deposit of chromium upon the surface of the article. There have also been difficulties in securing a condition in the bath which will secure reliability in the electrolytic action.

In most baths heretofore used there is a strong ebullition of noxious fumes making the depositing operation somewhat hazardous to workers.

In some instances it is found that while an apparently satisfactory chromium surface is secured, after a short interval the chromium plating will strip or fleck from portions of the surface due to corrosion of the material upon which the chromium is deposited beneath the plated surface.

Great diiiiculties have also been experienced in securing a uniform deposition of the chromium throughout the entire surface.

With the above conditions in mind, I have developed a process for theelectro-deposition of chromium in which the ingredients of the bath are of a character which will not only avoid the presence of fumes injurious to the workmen, but which will offer .a low, substantially uniform resistance to the flow of the electric current, thus avoiding those difficulties arising from a failure, during the process. to continuously deposit the chromium at a uniform rate due to the development of excessive resistance throughout, or in different parts of the bath, whatever the cause of such phenomena may be.

A bath used in the process of my invention is ready for immediate useupon the mixing of the solution, and the process may be carried on at ordinary factory temperatures or at a slightly elevated temperature, according to the character of the surface desired, or the 1926. Serial No. 104,208.

rapidity with which it is desired to throw the chromium.

In electro-depositing chromium, the reactions are accompanied by a decomposition of the water of the electrolyte, and the presence of minute bubbles of hydrogen at the cathode, which interfere with the adherence of the reduced chromium salt. There is also, with some baths, a formation of a brownish film upon the cathode which must be removed therefrom at the completion of the plating process. In the bath used with the process of my invention, the cathode is kept free of hydrogen gas bubbles, and the deposited chromium, when the cathode is removed from the bath, is in a clean, bright condition without any evidence of the brown scum above referred to, thus requiring a mere washing of the cathode to clean it.

I find it essential to thoroughly clean the surface of the cathode before immersing it in the bath, and to protect the surface thereof from the action of oxygen thereon with a film of clear water, before so immersing it in the bath.

In the practice of the process of my invention, I form a bath containing chromic acid (CrO and sodium compounds such as sodium hydroxide (NaOH), sodium sulfate (Na SO' and sodium carbonate (Na cO in proportions of substantially 95 parts NaOI-I, 2 parts Na SO and 3 parts Na CO- These compounds are placed in a solution of chromic acid, and in mixing the bath, to each gallon of water, I add about 8 ounces of the sodium compounds for at least 16 ounces of chromic acid.

The commercial form of sodium hydroxide which appears in commerce as lye is used. This substancecontains sodium sulfate and sodium carbonate as impurities.

While it is possible to use either a lead or a chromium anode, I have found that the latmetallic chromium may be immediately secured with the use of an electric current at a voltage of from 5 to 10, and an an'iperage of from 1 to2, or more, per square inch of the cathode.

If it be found that the resistance of the bath is so high as to reduce the current density at the cathode, ll may add in small quantities, further amounts of chromic acid until the proportions thereof in the bath, in relation to the sodium salts, will be from 2 to 3 to 1.

The use of sodium salts will not only reduce the resistance of the electrolyte, but during the reactions adjacent the cathode apparently form combinations with the hydrogen, thus causing the sodium to re-combine with the water of the bath. Of course the caustic soda will have the effect of keeping the cathode clean in respects other than avoiding the accumulation of hydrogen bubbles upon and adjacent the cathode. The sodium salts also appear to prevent the colloidal condition of the solution at the cathode, since there is an absence of the brownish. colloidal film upon the surface of, a cathode when removed from the bath.

' In a bath containing sodium, the only metallic salts which may be reduced and thrown upon the cathode are those contained in the chromic acid, and hence the deposited surface Wlll consist of pure chromium deposited directly against the metal of the cathode.

By using the higher temperatures in the bath as stated above, the hardness of the depos1t and the brilliancy of its finish are increased. With a lower temperature bath, the surface will be somewhat duller and will also be somewhat softer. Under any circumstances, however, the deposited chromium wlll be very hard, and when a bright finish s secured in the bath, no subsequent polishing or buffing is required. Ordinarily in plating with chromium. a deposit of from .0001 to .0002 of an inch is all that is required.

With the process of my invention, a flash of chromium may be applied to the cathode instantly upon placing it within the electrolytic bath, and while the time interval may vary with different temperatures in the bath very excellent results can be secured by in tervalsof from twenty minutes to one hour. Of course ifit is desired to increase the thickness of the deposit, this time interval may be ncreased, there being no tendency of polariaation of the bath, or an increase of its resist'ance, with a prolongation of the time interval. I

If 'it be desired to remove the chromium surface from the cathode, this may be done by using ordinary well known solvents for chromium, or by placing the cathode in a bath and reversing the direction of the flow of the current, or using the plated article as the anode.

The thorough cleaning of the cathode before its immersion in the bath is necessary to the production of a chromium surface thereon of the desired fine texture and continuity. Tn cleaning the cathode, T immerse same in an alkaline bath, using it as a cathode of an electrolytic couple, the anode of which may be carbon plates or graphite rods. The voltage andamperage used is the same as that used in the plating bath. A very short time is required to thoroughly cleanse the cathode, three minutes having been found ample. The cathode is then removed from the cleaning bath, thoroughly rinsed in clear water, and immersed in the plating bath, the handling of the plate being done without disconnecting it from the electrical source.

The process of my invention may be used in surfacing flat or irregularly shaped articles, chromium plating being particularly desirable as a surfacing for articles Which require frequent cleaning or are subject to corrosion as a result of exposure. The hardness of the chromium permits it to be cleaned with the use of various detergents and abrasives, since such will have no effect whatever upon the surface except the removalof superficial matter therefrom. The cleaning action need not be accompanied by any polishing, since the removal of superficial deposits thereon will merely expose the finish of the surface, which will not become dim or dull by oxidation, corrosion or otherwise.

Chromium surfacing is particularly desirable in plumbing fittings, in fittings for boats, ships and automobiles, in master plates used in engraving machines, in copper and steel printing plates, and in various other articles wherein resistance to wear, or the avoidance of corrosive action of the elements thereon are matters of importance. A. chromium surface while very attractive in appearance by reason of its blue-gray and other color effects, nevertheless possesses advantages apart from its pleasing appearance.

Notwithstanding the hardness of the chromium surface, the article may be readily formed after the chromium surface has been applied thereto, without the formation of droxide to the bath will contribute largely toward an increased conductivity of the bath and uniformity in the resistance condition irrespective of the age of the bath or its extent of use.

What effect the sodium has in the reducing of the chromic acid, I have been unable to determine. Its chief utility appears to be the reduction of the resistance of the electrolyte, the reduction of surface tension at the cathode resulting from either the rapid separation of hydrogen bubbles from, the cathode or the combination of hydrogen with the oxygen of the sodium salt in a manner to avoid the presence of free hydrogen upon or adjacent the cathode and preventing the depositing of the colloids upon the cathode.

It is possible that the sodium sulphate assists in the decomposition of the anode, which would be a very slow, gradual action by rea son of the low content of this constituent of the bath.

The solution is not only simple as to the nature of the ingredients entering, thereinto, but requires no acquired skill in the forming of the bath or in its upkeep.

While certain gases are evolved during the electrolytic action, the volume of such is small, and the effect of same upon the worker is not at all serious, even if the precaution is not taken to provide a hood and vent, or other methods for dissipating these gases.

While I have disclosed specific operating conditions and certain proportions of the ingredients and relative proportions of the chromic acid to the sodium compounds, it Will be understood that'these are merely illustrative and that the proportions of the bath constituents will vary with the degree of purity and composition of the constituents used and that these variations in the constituents or the bath concentration will vary the operation conditions such as rate of deposit, current density and temperature and that variations may be made without departing from the spirit of the invention.

Having described the invention, what I claim as new and desire to have protected by Letters Patent, is

being at least two to one.

2. A chromium deposition bath characterized by being'the result-of mixing a water solution of chromic acid and inorganic so dium compounds, comprising caustic soda to the extent of substantially 95% of said sodium compounds, the ratio of the chromic acid to the sodium compounds being upward of two to one.

3. A chromium deposition bath characterized by being the result of mixing a water solution of chromic acid and inorganic sodiumcompounds, the sodium compounds comprising sodium hydroxide and sodium sulphate, the sodium hydroxide composing practically all of the sodium compound mixture, the bath containing suflicient chromic acid relative to the sodium compounds to make the bath, as a whole, acid, the chromic acid and the-sodium compounds mixture having a relation of at least two to one by weight.

4. A chromium deposition bath characterized by being theresult of mixing a water solution of chromic acid and inorganic sodium compounds comprising a mixture of sodium hydroxide, sodium sulphate and sodium carbonate, the sodium hydroxide composing nearly all of said sodium compounds mix- 

